The present invention relates to a solder work material constituted by either a solder paste composition or a resin flux-cored solder for use in soldering electronic devices on the surface of a circuit board, the solder work material being characterized in that the range of temperatures in which the solder work material can be employed without troubles is relatively wide (for example, in the range of xe2x88x9240xc2x0 C. to +85xc2x0 C.), and in that it can be employed in an outdoor telecommunication equipment having electronic devices, in particular, a ceramic capacitor, a film capacitor, etc. mounted thereon without giving rise to the generation of malfunctioning of circuit due to voltage noises. The present invention also relates to a circuit board having a residual film of flux left thereon after the deposition of the solder work material.
An electronic devices-mounted circuit board which is generally employed in an electronic equipment is constructed such that electronic devices such as capacitors, resistors, etc. are electrically connected with each other and fixed to a printed circuit board by means of soldering.
In particular, if surface-mounting electronic devices are to be soldered to a printed circuit board, a reflow soldering method is employed, wherein a solder paste is coated at first on each of solder lands, an electronic chip is then mounted on each of solder lands, and the solder paste is heated to melt the solder particles of the solder paste, thus soldering the electronic chip thereon.
As for the solder paste, a paste-like composition comprising solder particles and a flux is generally employed. As for the materials for this flux, there has been generally employed a composition comprising rosin or a rosin-modified resin as a basic component; a little quantity of additives including an activator such as amine halogenosalt (such as amine hydrochloride) or an organic acid, a thixotropic agent such as cured castor oil, and other kinds of materials which are needed depending on the end-use and dissolved in a solvent, if required.
The rosins including rosin and a rosin-modified resin are generally excellent in electric insulating property, in humidity resistance, and in solderability, so that they have been employed as a flux for a long time. Therefore, the rosins including rosin and a rosin-modified resin are frequently employed as a resin component of a binder for solder paste. For this reason, a solder paste is generally manufactured by mixing solder particles with a flux comprising rosins.
By the way, the printed film of a solder paste which is designed to be formed on the soldering lands of a printed circuit board is heated in the process of reflow soldering process to thereby solder the electrodes of electronic devices onto the soldering lands. The residue of flux (hereinafter referred to also as flux residue) in the solder paste that will be separated on the occasion of the soldering is left remained between the soldered portions and the surface of the printed circuit board or between the soldered electronic devices and the surface of the printed circuit board. When a ceramic capacitor is employed in an electronic chip constituting the electronic devices, since the ceramic capacitor contains BaTiO3 as a main component, voltage noises may be generated due to the piezoelectric effect that may be brought about as a mechanical pressure is applied to the ceramic capacitor. Further, because of the fact that the glass transition point of the resin in the conventional flux is fairly high (20-50xc2x0 C.), the flux residue that has been left between the surface-mounted ceramic capacitors and the surface of the printed circuit board becomes hard and brittle as it is exposed to a cold environment of freezing temperature or lower, so that when the temperature of the resin is lowered to the glass transition temperature or less due to the cold environmental temperature, the volume of the flux residue is easily permitted to expand. Once the volume of the flux residue is expanded, it will lead to the generation of strain in a laminated ceramic capacitor element that has been mounted on the printed circuit board. Further, due to this strain, voltage noises are caused to generate, thereby giving rise to the malfunction of the circuit (circuit malfunction) that has been constructed by the incorporation of this ceramic capacitor element into the printed circuit board. Generally, there is a strong tendency in the case of ceramic electronic devices that voltage noises as mentioned above would be easily caused to generate. When ceramic electronic devices are employed in an outdoor electronic communication equipment, the aforementioned phenomenon of voltage noises is more likely to be generated due to an increased fluctuation of temperature thereof, thereby possibly raising a serious problem which may directly lead to communication troubles.
In recent years, due to an increasing tendency to further miniaturize and to further densify electronic devices, the circuit wirings is also increasingly densified, thus resulting in the miniaturization in pitch of soldering lands. As a result, the parasitic capacity due to the flux residue on the surface of printed circuit board is caused to increase, so that the voltage noises due to the fluctuation in volume of the flux residue tends to easily generate. Namely, this problem of voltage noises is more likely to be generated in a printed circuit board where the pitch of soldering lands is narrowed (not more than 0.3 mm), thereby frequently resulting in an increase in the scope of obstruction.
Therefore, with a view to minimize the influence of the residual film of flux, a washing treatment using a cleaning agent (water, an organic solvent, a surfactant, etc.) for washing the residual film of flux after the soldering has been performed on an electronic devices-mounting printed circuit board to be employed in an electronic equipment which is designed to be used even in a cold environment of freezing temperature or less. However, such a washing treatment necessitates a large quantity of water or organic solvents, thereby raising not only a problem in terms of protecting global environment but also a problem of increased manufacturing cost due to the provision of washing process. Further, since the performance of electronic devices tends to alter by the washing with the aforementioned cleaning agent, the washing treatment is technically restricted in that it cannot be applied to surface-mounting devices which are not suited for such a washing treatment.
With a view to overcome the aforementioned problems, it is now desired to develop a solder paste useful in reflow soldering, which is capable of preventing the volumetrical expansion of the residual film of flux even under a wide range of temperatures even if the residual film of flux is left remained on an electronic devices-mounting printed circuit board without performing the washing treatment.
The aforementioned desire is also applicable likewise to the xe2x80x9cresin flux-cored solderxe2x80x9d, i.e. a solder work material which can be obtained by a process wherein a flux for resin flux-cored solder which has been thermally fused or dissolved in advance, is poured into a hollow body of soldering ground metal, which is then stretched by means of drawing machine until it becomes a wire of a predetermined diameter to thereby obtain the aimed solder (i.e. a cored solder whose axis is filled with the resin flux). By the way, this cored solder is generally employed for soldering electronic devices which are incapable of withstanding the heating temperature of reflow in the reflow soldering method onto a printed circuit board after other kinds of electronic devices have been reflow-soldered on the printed circuit board (post-soldering), or for post-soldering electronic devices which cannot be automatically mounted on the printed circuit board, or for correcting the failure of reflow soldering, if any.
Therefore, a first object of the present invention is to provide a solder work material for forming solder-coated circuit boards, which is capable of preventing the malfunction of circuit that may be caused to generate by voltage noises that tend to be generated by the volumetrical expansion of the flux residue in a soldered laminated ceramic capacitor, etc. even under the environments where temperatures fluctuate widely (for example, xe2x88x9240xc2x0 C. to +85xc2x0 C.), or that may be caused to generate by voltage noises to be generated by the fluctuation of parasitic capacity between circuits which may be caused to generate due to a flux residue. A further object of the present invention is to provide a circuit board having a residual film of flux left thereon after the deposition of the solder work material.
A second object of the present invention is to provide a solder work material which can be manufactured in the same manner as in the case of the conventional paste containing rosins and which can be used in the same manner as the conventional circuit boards. A further object of the present invention is to provide a circuit board having a residual film of flux left thereon after the deposition of the solder work material.
A third object of the present invention is to provide a solder work material which makes it possible to obtain a residual film of non-wash type flux enabling the aforementioned first and second objects to be achieved. A further object of the present invention is to provide a circuit board having a residual film of flux left thereon after the deposition of the solder work material.
With a view to solve the aforementioned problems, the present invention provides: (1) a solder work material for forming solder-coated circuit boards, which is useful in soldering electronic devices onto a circuit board, and is constituted by either a solder paste composition or a resin flux-cored solder, both comprising solder particles and a flux containing at least a resin component; wherein said resin component comprises acrylic resin, and a rosin-based resin, and is useful in preventing the generation of voltage noises of electronic devices.
The present invention also provides: (2) a solder material for forming solder-coated circuit boards as set forth in the aforementioned item (1), wherein said resin component comprises 33 to 40% by weight of acrylic resin, and 12 to 18% by weight of a rosin-based resin.
The present invention further provides: (3) a solder work material for forming solder-coated circuit boards as set forth in the aforementioned item (1) or (2), wherein said acrylic resin is a copolymer comprising 10 to 20% of acrylic acid, 30 to 50% of acrylate, and 30 to 50% of methacrylate.
The present invention further provides (4) a solder work material for forming solder-coated circuit boards as set forth in the aforementioned item (1) or (2) or (3), wherein said acrylic resin is 4000 to 50000 in molecular weight; 25 to 400 in acid value; and xe2x88x9250xc2x0 C. to xe2x88x9240xc2x0 C. in glass transition temperature.
The present invention further provides (5) a circuit board having electronic devices mounted thereon and having a residual film of solder work material left thereon after a soldering process using the solder work material set forth in the aforementioned item (4).
The present invention further provides (6) a circuit board as set forth in the aforementioned item (5), wherein voltage noises to be generated by a strain of surface mounting electronic devices due to volumetrical expansion of the residual film of flux are prevented.
The present invention further provides (7) a circuit board as set forth in the aforementioned item (5), wherein voltage noises to be generated by the fluctuation of parasitic capacity among circuits due to volumetrical expansion of the residual film of flux are prevented.
The present invention further provides, in addition to the inventions set forth in the aforementioned items, xe2x80x9ca solder paste composition for forming solder-coated circuit boards, which is useful in soldering electronic devices onto a circuit board, and is constituted by a solder paste comprising at least solder particles and a flux for solder paste; wherein said flux comprises at least a resin component and a solvent component, and said resin component comprises acrylic resin (or acrylic resin and rosin-based resin).xe2x80x9d. This invention may be further limited by any of the aforementioned items (2) to (7).
The aforementioned expression of xe2x80x9csolder work materialxe2x80x9d may be replaced by other expressions such as xe2x80x9ca solder paste composition for forming solder-coated circuit boardsxe2x80x9d, xe2x80x9ca solder paste composition for reflow solderingxe2x80x9d, or xe2x80x9ca solder paste composition for forming reflow solder-coated circuit boardsxe2x80x9d. Further, the aforementioned expression of xe2x80x9csoldering methodxe2x80x9d may be replaced by an expression of xe2x80x9creflow-soldering methodxe2x80x9d. The aforementioned expression of xe2x80x9cis useful in preventing the generation of voltage noises of electronic devicesxe2x80x9d may be replaced by an expression of xe2x80x9cis useful in preventing the generation of voltage noises of electronic devices through the inhibition of volumetrical expansion, under a low temperature, of flux residue after solderingxe2x80x9d.